Pressure vessels, such as those found in power plants, are provided with a variety of access openings ranging from small hand holes to large reactor vessel covers. The closures must withstand high pressures and usually high temperatures. Optimum closure integrity is achieved through uniformly distributing the load about the closure by uniformly tensioning all of the studs used to support the closure about the access opening.
Off-the-shelf stud tensioning devices include multiple stud tensioning rings which tension all or some of the studs simultaneously. These devices are brought in when needed and removed after use. In some applications, a load cell can be installed permanently about each stud for use when needed.
However, many studs within power plants are subjected to temperatures of up to 1000.degree. F. (538.degree. C.) during operation of the plant. Therefore, permanent load cells are ruled out as they employ rubber or polymer materials for sealing purposes. Also, many of the studs in fossil power plants are of a large bolt diameter. Use of a multiple stud tensioning device or a portable load cell is not always feasible since tensioning of large diameter bolts requires relatively high forces and the spatial constraints associated with power plants may deny access to the large equipment required to deliver these large forces.
An exemplary large bolt diameter which may reach 1000.degree. F. (538.degree.0 C.) during operation is employed on the high pressure/intermediate pressure (HP-IP) turbine cylinder. These cylinders must be opened and resealed once or twice in a five year period. The studs on the cylinder closure must be prestressed to 45,000 psi (3.17.times.107 kg/m.sup.2) as the nuts are installed. The hydraulic force on each stud is then relaxed, resulting in a uniform stress level in all studs about the closure. Stud tensioning is not again required until the next opening of the cylinder. Due to the large bolt diameter of the studs used on the HP/IP cylinder closure, no commercial load cells or multiple stud tensioning devices are available. Stud tensioning is presently performed from above by pulling with a hoist. This method of tensioning the studs is extremely time consuming and labor intensive.
What is needed is a device which can be permanently mounted about each stud for prestressing of the studs about the closure and for subsequent stressing of the studs for removal of the closure when necessary. Such a device must be able to tension large diameter bolts and survive an environment which reaches 1000.degree. F. (538.degree. C.).